Electrically switchable structures also referred to as “intelligent” glazing structures, or “smart windows”, have been used to control electromagnetic radiation in buildings and vehicles. Such structures have light transmission characteristics that can be electrically controlled during the course of the day, or year, in order to meet lighting needs, minimize thermal load on heating and/or cooling systems, and provide privacy within the interior spaces of buildings and vehicles.
There are two general categories of chromogenic switchable glazing or smart windows, namely: non-electrically activated switchable glazings and electrically activated switchable glazings. The non-electrically activated types of chromogenic switchable glazing are based on photochromics, thermochromics and thermotropics. The most common electrically activated types of chromogenic switchable glazing are based on polymer dispersed liquid crystals (PDLC), dispersed particle systems (DPS) and electrochromics.
Electro-optical laminate structures having total-reflection, semi-transparent and totally transparent modes of operation for improved control over the flow of electromagnetic radiation have been developed. Such structures comprise one or more cholesteric liquid crystal (CLC) electromagnetic radiation polarizing panels.
CLC polarizers are used in light valves and electro-optical glazing, or smart window constructions to control light. Such constructions typically comprise two rigid sheets of glass on either side of the CLC layer. The CLC layer comprises crosslinkable or polymerizable material mixed with non-crosslinkable liquid crystals and chiral dopants. Each sheet of glass is covered with a transparent, electrically conductive coating to which electrical connections are attached. The structure is typically mounted within a frame.
In the “normal” mode, the CLC layer appears opaque. The liquid crystals are oriented in multiple directions and scatter light striking the CLC layer, making the device appear opaque. When the device window is switched on, the electrical field between the two conductive coatings forces the liquid crystals to reorient themselves parallel to each other. The CLC layer then appears transparent, and light passes through the device without scattering. U.S. Pat. Nos. 5,437,811 and 5,691,795, and International Publications WO 93/23496 and WO 0060407 describe electro-optical structures that operate in the “normal” mode.
Electro-optical devices incorporating CLC polarizers may also be configured to operate in “reverse” mode, wherein the device initially appears clear and is switched to opaque. When no electrical field is applied to the CLC layer, light passes through the device without scattering. Upon application of the electrical field, the liquid crystals reorient themselves to scatter light. U.S. Pat. Nos. 5,437,811 and 5,691,795, and International Publication WO 93/23496 describe electro-optical structures that operate in the “reverse” mode.
Electro-optical laminate structures may also be configured to operate in “reflective” mode, wherein the device is electrically switched between low and high reflectivity. U.S. Pat. Nos. 5,251,048; 5,384,067; 5,668,614; 5,940,150 and 6,072,549 and International Publications WO 98/38547 and WO 99/63400 describe electro-optical structures that operate in “reflective” mode.
In “bistable” mode, the liquid crystals are stable in both the clear state and the scattering state. The electro-optical structure requires electrical power only during switching. Power is not required to maintain either the clear state or the scattering state. U.S. Pat. Nos. 5,691,795 and 5,748,277 describe “bistable” electro-optical structures.
There is a need for switchable electro-optical devices improved optical properties and increased stability. Additionally, there is a need for electro-optical laminate structures that can be readily customized to fit various applications.